The present invention relates in general to base radiocommunication systems and, in particular, to channel allocation combined with power control in a mobile radio communication system.
The concept of frequency reuse is at the heart of cellular technology. In the conventional sense, frequency reuse is a technique whereby groups of frequencies are allocated for use in regions of limited geographic coverage known as cells. Cells containing equivalent groups of frequencies are geographically separated to allow callers in different cells to simultaneously use the same frequency without interfering with each other. By so doing many thousands of subscribers may be served by a system of only several hundred frequencies. The design and operation of such a system is described in an article entitled Advanced Mobile Phone Service by Blecher, IEEE Transactions on Vehicular Technology, Vol. VT29, No. 2, May, 1980, pp. 238-244. Known commonly as the AMPS system, this system had allocated to it by the FCC a block of the UHF frequency spectrum further subdivided into pairs of narrow frequency bands called channels. Pairing results from the frequency duplex arrangement wherein the transmit and receive frequencies are offset by 45 MHz. At present there are 832, 30 kHz wide, channels allocated to cellular mobile communications in the United States. A table of the frequencies dedicated to mobile communications in the U.S. is shown in FIG. 1. It is worth noting at this point that of the 832 available channels, there are 21 control channels dedicated each to the A-carrier and the B-carrier. These 42 control channels provide system information and cannot be used for voice traffic. The remaining 790 channels, known as voice or traffic channels, carry the burden of voice communication and are equally divided between the A-carrier and the B-carrier. A particular user can access at least half of the available channels, or 395. With regard to TDMA systems, such as specified by the IS-54B standard, these channels are further divided into 3 time slots. In this instance, a given user can access 3.times.395, or 1185, "channels".
Link quality is the benchmark of any radio communication system. To provide high quality voice communication the desired signal in a cellular system must maintain a minimum signal strength above all other interference. The ratio of the desired signal to the interference is known as C/I. Aside from noise, which is omnipresent, there are fundamentally two other types of interference with which a designer must contend. The first of these is interference arising from users simultaneously operating on the same channel. This is known as co-channel interference. The second source of interference is from users operating on adjacent channels. This is known as adjacent-channel interference. Adjacent channel interference is controlled by selecting the frequencies in a given cell to be separated by large frequency increments, e.g., 200 kHz, and by using sharp cutoff in the channel filters in order to obtain a high adjacent-channel suppression. Co-channel interference is reduced by use of a frequency reuse pattern which geographically separates cells with the same frequency group. An example of an ideal seven cell frequency reuse pattern is shown in FIG. 2(a).
Frequency planning is the process by which individual channels are assigned to cells within the network. Currently, most frequency planning is done a priori; that is a fixed frequency plan is "hard-wired" in place by each cellular system operator. This is known as fixed channel allocation, or FCA. However, as interference and traffic load are time varying, FCA is not optimal. As shown in FIG. 2(b), highways which bisect cellular boundaries may have significantly differing traffic patterns depending on location and time of day. Some roads may have significant automobile traffic in the morning and very little in the afternoon. As a result, most fixed frequency plans are not very efficient; many channels in a fixed frequency plan will have a much better link quality than is necessary to achieve high quality voice communication while many others in the same system will suffer from poor link quality which might force them to be dropped or blocked. A capacity increase could be obtained by some form of channel allocation where all of the links have equal quality. Because of the time varying nature of the interference, an adaptive scheme must be used.
Adaptive channel allocation, or ACA, is a method of dynamically allocating frequencies throughout a cellular system to maximize system capacity. Under an ACA scheme, more frequencies would be allocated to busy cells from more lightly loaded cells. In addition, the channels can be allocated such that all links have satisfactory quality.
The concept of ACA is well-known to those skilled in the art. Many publications have illustrated the potential for ACA yet do not discuss specific strategies. For example, "Capacity Improvement by Adaptive Channel Allocation", by Hakan Eriksson, IEEE Global Telecomm. Conf., Nov. 28-Dec. 1, 1988, pp. 1355-1359, illustrates the capacity gains associated with a cellular radio system where all of the channels are a common resource shared by all base stations. In the above-referenced report, the mobile measures the signal quality of the downlink and channels are assigned on the basis of selecting the channel with the highest C/I level.
Another approach is described by G. Riva, "Performance Analysis of an Improved Dynamic Channel Allocation Scheme for Cellular Mobile Radio Systems", 42nd IEEE Veh. Tech. Conf., Denver, 1992, pp. 794-797 where the channel is selected based on achieving a quality close to or little better than a required C/I threshold. Furuya Y. et al., "Channel Segregation, A Distributed Adaptive Channel Allocation Scheme for Mobile Communications Systems", Second Nordic Seminar on Digital Land Mobile Radio Communication, Stockholm, Oct. 14-16, 1986, pp. 311-315 describe an ACA system wherein the recent history of link quality is considered as a factor in allocation decisions. In addition several hybrid systems have been presented where ACA is applied to a small block of frequencies on top of an FCA scheme. Such an example is presented in Sallberg, K., et al., "Hybrid channel assignment and reuse partitioning in a cellular mobile telephone system", Proc. IEEE VTC '87, 1987, pp. 405-411.
A common denominator for all of these ACA systems is that they allocate a channel out of the set of channels which fulfills some predetermined quality criteria. The difference in each is how the channel is chosen out of the set. Apart from increases in system capacity, adaptive channel allocation obviates the need for system planning. Planning is instead performed by the system itself; this is particularly attractive when system changes are implemented or new base stations are added.
Adaptive power control, or APC, is also a known art in cellular systems. See, for example, U.S. Pat. No. 4,485,486 to Webb et al. With APC, the power of the transmitter is varied according to the needs in the receiver. In general, there are two types of adaptive power control schemes: the C-based and the C/I-based. In the C-based scheme, the signal strength level at the receive side is maintained at a predefined level. As soon as the (average) received signal strength deviates from this level, the transmitter is ordered by the receiver to increase or decrease its transmit power. C-based APC only responds to changes in the path loss whereas C/I-based APC tries to maintain a predefined C/I level at the receiver. In addition to changes in the path losses, changes in the interference condition also result in the transmit power being adjusted.
The conventional allocation algorithms described above base their decisions on the knowledge of which channels are used by which base stations and then attempt to optimize the quality in each link. However, they do not take advantage of the possibilities offered by adaptive power control in the mobiles and base transmitters.
In a TDMA environment, the allocation decision includes more than just a selection of the base and channel combination. Since TDMA channels are broken up into time slots, the allocation decision should also take this into account. U.S. Pat. No. 4,866,710 to Schaeffer, for example, describes a method of allocating frequency and timeslots to mobile stations such that all the timeslots on a given frequency are filled before allocating timeslots on another frequency. Although seemingly efficient, this scheme does not consider the contributions to interference and does not consider the possibility of adaptive power control.